One of the conventionally employed methods for producing a glass preform is a so-called OVD method (cf. Japanese Patent Kokai Publication (unexamined) Nos. 73522/1973 and 18909/1974). The OVD method comprises oxidizing the glass raw material such as SiCl.sub.4 in an oxidizing flame to form fine glass particles of, for example, SiO.sub.2 and depositing the fine glass particles in a predetermined amount on a seed rod (e.g. a preinstalled core glass rod, a supporting mandrel which is to be removed before or after sintering of the glass preform, etc.) to form a soot rod of the fine glass particles and then sintering the soot rod in a suitable atmosphere to obtain a transparent melt glass preform for an optical fiber. According to the conventional OVD method, the highly pure glass preform is produced in a comparatively high productivity, and dehydration of the glass preform and/or addition of an additive is effectively carried out during sintering since the deposition and sintering steps are separately performed.
However, the conventional OVD method has some drawbacks, that is, if a surface temperature of the soot rod is low in the deposition step, the soot rod tends to crack while the deposition yield of the fine glass particles is improved. If the surface temperature of the soot rod is high, the deposition yield is decreased while the soot rod does not crack.